1. Field of the Invention
The present invention relates to a semiconductor device fabricating method and, more particularly, to a semiconductor device fabricating method capable of securing the flatness and reliability of a wiring layer formed over an oxide film by reducing the step of bird's heads in the underlying oxide film formed by a LOCOS process.
2. Description of the Prior Art
A LOCOS process, which oxidizes portions of a silicon substrate selectively to form thick oxide films locally, is an important process of isolating the devices of an LSI circuit and is effective for securing parasitic capacity, wiring capacity and field Vth. When a portion of a silicon (Si) substrate is oxidized into SiO.sub.2, the volume of the oxidized portion of the Si substrate increases. Accordingly, the SiO.sub.2 portion protrudes from the surface of the Si substrate to deteriorate the flatness of the surface of the Si substrate. A known recessed LOCOS process etches a Si substrate to form recesses corresponding to portions which are to be oxidized and oxidizes the recessed portions in order that the surface of the Si substrate is flat after the Si substrate has been subjected to the recessed LOCOS process. In the recessed LOCOS process, a SiO.sub.2 film 21 is formed by hot oxidation over the surface of a Si substrate 20, a Si.sub.3 N.sub.4 film 22 is formed over the SiO.sub.2 film 21 by a CVD process as shown in FIG. 4(a), portions of the Si.sub.3 N.sub.4 film 22, the SiO.sub.2 film 21 and the Si substrate 20 are removed selectively by mesa etching as shown in FIG. 4(b), and then, a LOCOS SiO.sub.2 film 23 is formed by high-temperature humid oxidation, and then the Si.sub.3 N.sub.4 film 22 and the SiO.sub.2 film 21 are removed to expose the device forming area of the Si substrate 20 as shown in FIG. 4(c).
Although the recessed LOCOS process finishes the surface of the Si substrate in a generally flat surface, bird's heads 24, i.e., protrusions, having a height W1 are formed along the edges of the LOCOS SiO.sub.2 film 23. These bird's heads 24 is liable to form breaks in a wiring pattern formed over the LOCOS SiO.sub.2 film 23, deteriorating the reliability of the associated device.